The present invention relates to the art of intrusion detection. More particularly, it relates to improvements in intruder detection systems of the so-called "dual technology" variety.
Heretofore, a variety of "technologies" have been used to detect the presence of an intruder in region under surveillance. Microwave, ultrasonic, photoelectric and passive infrared are some of the more common technologies in current use. Each has certain unique advantages and disadvantages which makes it more or less desirable for a particular environment or application. None is fool-proof, and all are subject to the ever-annoying false alarm.
In the never-ending quest to provide a "false alarm-proof" intruder detection system, proposals have been made to combine two (or more) technologies in a common intruder detection system. See, for example, the disclosures of U.S. Pat. Nos. 3,725,888; 3,801,978; 4,243,979; 4,275,390; 4,331,952; 4,401,976; 4,710,750 and 4,833,450. While such proposals go back some thirty years (see, e.g., U.S. Pat. No. 3,074,053), only recently has the cost of electronics reached a level that has made commercialization of a "dual-tech" system viable.
In conventional dual-technology systems, the outputs of the different intruder-detecting subsystems (e.g. microwave and passive infrared subsystems) are fed to an AND gate or its equivalent. Only in the event that the outputs of both subsystems indicate that both subsystems have detected intrusion substantially simultaneously, or within a predetermined, relatively short time interval, will the AND gate provide an alarm-activating signal. The advantage of such a system, of course, is that false alarms will only occur on the relatively rare occasion that spurious, false alarming-producing events are detected by both subsystems at about the same time. By combining relatively diverse technologies, e.g. microwave and photoelectric or passive infrared, the probability of such an occurrence can be minimized.
In the commonly assigned U.S. Pat. No. 4,660,024 issued in the name of R. L. McMaster, there is disclosed a dual technology intruder detection system which incorporates a supervisory circuit for monitoring the operating status of a microwave subsystem. In the event such subsystem stops transmitting microwave energy or otherwise experiences a malfunction which prevents it from detecting intrusion, a supervisory signal is produced. In addition to being used to annunciate the malfunction (e.g., by energizing a light-emitting diode), such supervisory signal serves to cause the system to "default" to a "single" technology detection system (i.e., the still functioning subsystem). In this manner, some measure of protection is provided until the operating status of the malfunctioning subsystem is restored. Note, without such a default feature, the AND gate circuitry prevent the dual technology system from alarming until the malfunction was corrected. This default feature is particularly useful in applications where the user cannot frequently or easily verify the operating status of the system.
In using dual-technology detection systems, it is common for the manufacturer or installer to adjust the sensitivity of each subsystem to a level substantially higher than the sensitivity commonly used in a comparable single-technology or "stand alone" system. The rationale is that, since each subsystem is usually immune to the false alarm-producing sources of the other subsystem, and since both subsystems must alarm simultaneously before a "true" alarm condition (i.e., intruder-produced) can be produced, there can be no disadvantage in setting the sensitivity of each subsystem at its limit. While each subsystem may well produce frequent false alarms, the AND circuitry of such dual-tech systems prevents these false alarms from producing a true alarm condition. While this philosophy may be sound in the case of dual-tech systems having no "default" capability, it can be problematic to dual-tech systems which do incorporate this feature. Specically, it has been observed that within a relatively short time interval after default occurs, the still-functioning subsystem, owing to its unusually high sensitivity setting and/or the normally "harsh" environment in which dual technology are commonly used, false alarms and thereby causes the overall system to alarm.